The invention relates to a method of converting radioactive waste into a substantially water-insoluble solid form suitable for transportation and temporary or interim storage. More specifically, the invention relates to a method of transforming calcined, high-level radioactive wastes into a water-insoluble solid product suitable for transportation and interim storage.
A typical high-level waste is the result of the neutralization with sodium hydroxide of a highly acidic nuclear fuel waste processing solution such as in the case of reprocessing of irradiated nuclear fuel. The stored waste typically consists of a supernatant liquid layer, which has a high concentration of sodium nitrate and nitrite salts, and a bottom insoluble layer of sludge. The precise composition of the supernatant liquid and the sludge are determined by the fuel and the subsequent processing details. The sludge obtained from reprocessing of fuel may consist largely of iron and aluminum hydroxides. In addition, iron phosphate, nickel and chromium hydroxides, aluminum fluoride, some rare earths, molybdenum phosphate, and manganese, plutonium and uranium oxides may be present in the sludge.
These high-level liquid radioactive wastes are often produced at facilities in which terminal processing of the waste is not practical because the quantities of waste are relatively small and the capitol cost of terminal processing equipment and facilities is high. Current regulations do not permit shipment of high-level liquid wastes. Under these circumstances the waste must be incorporated into an interim solid form for shipment to a terminal processing site. Because the shipment waste form is not expected to remain intact for more than a few years, the criteria for an acceptable interim waste form is not as stringent as that required for geological storage for a final waste form.
Ideally, the interim form should require only simple processing methods, so that extensive additional facilities will not be required at the originating site. Additionally, the interim waste form must satisfy shipping criteria, particularly in relation to transportation accidents. For example, the form must be resistant to impact fracturing and when fractured, should not produce large quantities of fine particles (&lt;10 .mu.m) which could become airborne and spread radioactivity into the environment. Furthermore, the form must be physically and mechanically stable in the ambient environment, should be able to withstand high temperatures without decomposition, should be sufficiently insoluble in water so that, when exposed (e.g. in case of an accident), would allow reasonable time to clean up without extensive loss due to leaching. The interim form should also be compatable with proposed terminal waste forms.
A number of interim forms for handling these high level radioactive wastes have been proposed. The simplest form is a fused-salt/sludge waste form, which is prepared simply by evaporating the water from the supernatent, melting the precipitated salts, slurrying the oxide and hydroxide sludge components into the melted fused salt and freezing the mixture to form cakes of sludge in fused sodium nitrate/nitrite salt. These cakes can then be shipped and redissolved or remelted for terminal processing. This fused-salt waste form has the advantage of simple processing and of allowing simultaneous treatment of the supernatant liquid and of the sludge. Synthetic fused salt/sludge solid samples were found to dissolve very rapidly in stagnant water at ambient temperature. Fused salt samples were also found to be highly hygroscopic. The samples were often partially or completely dissolved in the absorbed moisture, indicating that the waste form must be stored in a dry environment to avoid moisture absorption. The high water solubility and hygroscopicity of the fused salt/sludge interim waste form make its storage and transportation somewhat difficult. The storage canister must be able to overcome the difficulties of this waste form.
Another proposed form for handling high-level liquid radioactive wastes is the use of cement or concrete as a matrix for interim solidification. However, the primary difficulty of using these compositions (e.g. Portland cement) which often contain oxides of calcium, aluminum and smaller amounts of magnesium and iron, as an interim waste form is the incompatability of the cement or concrete with a terminal waste form such as borosilicate glass, which at present is the terminal waste form most likely to be used.
Still another suggestion is to blend calcined sludge with anhydrous amorphous sodium silicate powder, cold press the blended mixture to pellets, and subsequently sinter the pellets to high density by firing them to approximately 600.degree. C. in air. Anhydrous amorphous sodium silicate (with SiO.sub.2 :Na.sub.2 O ratios of 3.22 and 2.00) powders, in pure form, can easily be sintered to &gt;90% theoretical density. However, additions of synthetic sludge to sodium silicates were found to substantially reduce sinter density and fracture strength thus deteriorating the properties of the waste form. In addition, the sintering step would require use of a high-temperature furnace which adds substantial complexity to the process.
All of the above described interim waste forms for high-level radioactive waste, have limitations. Either they are incompatable with a proposed terminal waste form, are too soluble in water or require difficult processing steps for fabrication.